From 50 to 1200 mOsm per kilogram, needing an intact loop, gradient and hormone
Expansion
Urine osmolality ranges from about 50 to 1200 mOsm/kg.
To concentrate urine you need
- A functioning loop of Henle to build the gradient
- An intact medullary gradient, preserved by low vasa recta flow
- Antidiuretic hormone
- Responsive collecting ducts with aquaporin-2
Failure of any one causes polyuria: loop diuretics abolish the gradient, central diabetes insipidus removes the hormone, and lithium or hypercalcaemia cause nephrogenic resistance.
To dilute urine you need a functioning ascending limb (which removes solute without water) and the absence of antidiuretic hormone.
Obligatory urine volume: the daily solute load is about 600 mOsm, so at a maximum concentration of 1200 mOsm/kg the minimum urine volume is 500 ml. A low solute intake, as in “beer potomania” or a tea and toast diet, reduces the solute load and therefore the capacity to excrete water, which is why these states cause hyponatraemia.